In the field of optical communications, it is well known to use semi-conductor laser diodes to generate a narrowband optical signal onto which data is modulated for transmission through an optical medium such as an optical fibre link. In order to obtain desired characteristics of the optical signal (such as center wavelength, line width, signal reach, for example) the output power of the laser diode must be maintained within narrow tolerances. Because different laser diodes have different output power characteristics in response to a given driving current, it is desirable to monitor the output power from each laser diode, and adjust the driving current as needed to maintain the output power at a desired level. FIG. 1 schematically illustrates a feedback control loop 2 for this purpose.
In the feedback control loop 2 of FIG. 1, a semiconductor laser diode 4 is typically constructed as a multi-layer doped semiconductor structure defining a laser cavity in which light is generated in response to a drive current 6 supplied by a controller 8. In the case of a direct modulation transmitter, the drive current 6 will comprise a bias current and a modulation current derived from data being transmitted. In the case of an external modulation transmitter, the drive current 6 will typically comprise only the bias current. Reflective front and back facets 10, 12 define the respective front and back boundaries of the laser cavity. The front facet 10 is designed to be only partially reflective. Light emitted through the front facet forms the optical signal 14 onto which data is modulated for transmission. The optical power level of the optical signal 14 emitted by the front facet 10 is considered to be the output power of the laser diode 4.
By contrast, the back facet 12 is normally designed to be a highly reflective surface, so as to minimize “leakage” of light through the back facet 12, and thereby maximize the output power of the laser diode 4. However, the leakage of light through the back facet 12 is not zero, so that back facet light 16 leaks through the back facet 12 of the diode 4. The power level of the back facet light 16 is known to be proportional to the power level of the optical signal 14 emitted by the front facet 10. This relationship between laser output and back facet light 16 affords the opportunity to monitor the output power from the front facet 10 by detecting the back facet light 16.
Typically, a photodetector 18 is placed proximal the back facet 12 of the laser diode 4 to detect the back facet light 16 emitted through the back facet 12. The output current 20 of the photodetector 18 is proportional to the power level of the back facet light 16, and thus is also proportional to the output power of the optical signal 14 emitted through the front facet 10 of the laser diode 4. The controller 8 can then use various techniques known in the art, to control the output power of the laser 4 by adjusting the laser drive current 6 based on the monitored photodetector current 20. For this reason, the photodetector current 20 may conveniently be referred to as Back Facet Monitoring (BFM) current IBFM.
Back Facet Monitoring is commonly used for controlling non-injection seeded lasers, as described above with reference to FIG. 1. It would be desirable to also utilize Back Facet Monitoring to control injection seeded transmitters, including injection seeded lasers and reflective semiconductor optical amplifiers (RSOAs). As may be seen in FIG. 1b, an injection seeded laser 22 receives a seed light 24, which is used in combination with the drive current 6 to generate the output optical signal 14. However, in this case, the back facet light 16 emitted from the back facet 12 includes a first optical component 26 due to the drive current, and a second optical component 28 due to the seed light 24. Consequently, the BFM current 20 is highly dependent on the injection seed light power. This raises a difficulty in that the power level of the injection seed light 24 is unknown, and may change rapidly with time. As a result, conventional BFM techniques cannot be used to control injection seeded lasers. RSOAs suffer the same limitation, and thus cannot be controlled using conventional BFM techniques.
Techniques that overcome the above-noted limitations in the prior art remain highly desirable.